(1) Field of the Invention
In one aspect, this invention relates to a process for the manufacture of a rigid foamed polyurethane which does not use ozone-destroying cholorofluorocarbons (CFCs) as foaming agent. The CFCs also are believed to be so-called greenhouse gases. In another aspect, this invention related to the rigid foamed polyurethane manufactured by use of chlorodifluoromethane, CHClF.sub.2 (designated by HCFC-22) and chlorodifluoroethane, CH.sub.3 CClF.sub.2 (designated by HCFC-142b) as foaming agent which have no environmental problems because of the presence of hydrogen atoms in the molecules and the short life in the atmosphere.
The rigid foamed polyurethane of the present invention can retain its good insulating properties over a temperature of from about -30.degree. to -40.degree. C., and has less impairment in its thermal conductivity on standing while retaining its good flame resistance.
(2) Description of the Prior Art
It is known that rigid foamed polyurethanes made up substantially of closed cells, which are manufactured by use of a polyol, an isocyanate, water, CCl.sub.3 F (designated by CFC-11) as foaming agent, have good mechanical and thermal insulating properties, excellent dimensional stability, sound insulating and electrical properties, and chemical resistance. Accordingly, they are widely used in the making of thermal insulating materials for refrigerators, freezers, chill cars, iceboxes, containers for frozen food, LPG tanks; insulating materials for panels of curtain walls; building materials for roofing or canopies of cold storage warehouses; and various parts of automobiles.
Lowland of University of California disclosed a theory in 1974 that specific CFCs which include dichlorofluoromethane (CFC-11), dichlorodifluoromethane (designated by CFC-12), trichlorotrifluoroethane (designated by CFC-113), dichlorotetrafluoroethane (designated by CFC-114) and chloropentafluoroethane (designated by CFC-115), are suspected of being ozone-destroying substances. The CFCs have been linked to the cause of an Antarctic hole in the Earth's ozone layer, which screens skin cancer-causing ultraviolet radiation from the sun.
Further, the CFCs are said to have a long-life, for example, for about 100 years in the environment and they absorb infrared light. Therefore, the CFCs also are believed to be so-called greenhouse gases; substances that may contribute to global warming that could cause the earth's temperature to rise with potentially catastrophic effects for health and production of crops.
Their use is strictly regulated in technological advanced countries and tightened still further.
Their use and production have been regulated in Japan since 1989, and a complete ban on the CFCs production and use will be in effect in the near future. From this point of view, it is an urgent matter to develop substitutes for the CFCs.
CFC-11, which is included in the specific CFCs and used widely in making foamed plastics as foaming agent, is similarly circumstanced. The CFC-11 has a boiling point of 23.8.degree. C., and it liquefies in the closed cells of polyurethane when the polyurethane is used in making thermal insulating materials for refrigerators, freezers, or show cases which are maintained at about -30.degree. to -40.degree. C. In this case, the thermal conductivity of the insulating material changes sharply, yielding no effectual thermal insulating results.
Therefore, there is a need for developing a process for the manufacture of a rigid foamed polyurethane by use of a foaming agent which does not liquefy under such cold conditions and makes the foamed polyurethane retain its low thermal conductivity.